shy

_ LYCEN1944lt1 November 1994

ERMILAf IAN ~ 19(

ii~ NUCLEAR SPECTROSCOPY OF EXOTIC NUCLEI -I shy_ii~ WITH THE SARAIGISOL FACILITY

0 IBRAP-_0 ~o ~1 n ~~ ~ m -=-= ~

0 a ()lt 0 - - lgt NR Beraud1 A Emsallem1 J Arje2 A Astier1 J Aysto2 ~ -

~~~LJ --- D~ Barneoud3 R DyuffLaitCl J GNeneRvedy3 n~middotyGVizonS3h PmiddotkJaukh~2-lt -CX)

CD sect

I u A Lazarev e oz _ eo 1r0 OVS Y CD ~ tD

=0

1 Institut de Physique Nucleaire de Lyon IN2P3-CNRSUniversite Claude Bernard 43 Bd du 11 Novembre 1918 69622 Villeurbanne Cedex France

2 Department of Physics University of Jyvaskylii PO Box 35 SF-40352 Jyviiskylii Finland

3 Institut des Sciences Nucleaires de Grenoble IN2P3-CNRSUniversite Joseph Fourier 43 Avenue des Martyrs 38026 Grenoble Cedex France

4 GN Flerov Laboratory of Nuclear Reactions Joint Institute for Nuclear Research Moscow District Russia

Invited talk presented at

Third IN2P3-RIKEN Symposium on Heavy Ion Collisions Oct 24-28 1994 Osato-Gun (Japan)

I Introduction

The study of nuclei far off the valley of stability offers the possibility to extend nuclear structure data as a function of isospin so that more stringent tests of nuclear model predictions become possible

We review our recent studies on alpha and beta decay of exotic nuclei performed with the IGISOL on-line mass separator at SARA in Grenoble The experiments using charged particle induced fission have given new information on production cross-sections [1] and properties of n-rich nuclei with A==110-130 whereas by means of HI-induced fusion evaporation reactions we have investigated two regions close to the proton drip line around A=180 and A=130

One of the physics goal of our investigations was to understand the gradual change of nuclear structure from a spherical vibrator to a deformed rotor when going trom doubly magic 132Sn to the highly deformed lOOZr In fact we have found a transition region with rather strongly deformed nuclei among the most n-rich Ru isotopes with a breaking of axial symmetry [2]

The discovery of EC 3+-delayed fission of ultra-neutron deficient isotopes in the region of Mercury-Lead [3] and the lack of data on their decay properties were mainly at the origin of our experimental programme on the TI isotopes New insight on fission barrier of cold nuclei may be gained from these investigations

The n-deficient rare-earth nuclei with A around 130 are well-known to belong to a so-called transitional region where quasi-neutrons and quasi-protons may occupy h U 2 shells From in beam studies a lot of defo~med and superdeformed bands have been observed but their connection with the low-lying levels is often an open problem Its solution can be found thanks to a precise knowledge of level schemes at low excitation energy which can be obtained from 3-decay studies [4] Our most recent experiments involving also search for proton-radioactivity will be reported

In this paper we first give a brief description of the IGISOL (Ion Guide base Isotope Separation On Line) technique and show its applications in case of two different production modes charged particle-induced fission and HI-induced fusionshyevaporation reactions

II Experimental techniques

Since 1982 the ISOL method has been applied at SARA in Grenoble for the study of nuclei produced via heavy-ion (HI)-induced fusion-evaporation reactions Initially developed with an integrated target-ion-source [5] suitable for volatile eleshyments the method has been extented to refractory elements by connecting a He-jet to a medium-current source [6] This allowed us to overcome the problem of delay time from the catcher material which was a severe limitation to the study of isoshytopes with half-lives shorter than a few tens seconds in case of non-volatile species Thus a large number of n-deficient isotopes have been identified and studied in the light rare-earth region In this technique the delay time which is approximately the

2

transit time in the capillary of the He-jet system (typically a few tens msm) cannot be avoided and moreover taking into account the maximum He flow rate compatible with the working pressure of the ion source it was not possible to go much lower than 1s range

A decisive progress was made with the IGISOL technique pioneered by the Jyvaskyla group [7] in which the delay time between production and detection afshyter mass separation is mainly due to the mean evacuation time of the He-pressurized recoil chamber This group has shown that in the case of light ion-induced fusionshyevaporation reactions the overall transmission efficiency can be of the order of 1 even for isotopes with short half-lives down to 100 JLS [8] In addition the IGISOL technique has very stable operating conditions and its chemical non-selectivity makes it very attractive for refractory elements Its basic principle takes advantage of the fact that nuclear reaction products recoiling out of a target and thermalized in He may survive as singly charged ions for about 10 ms This time is usually long enough to extract the ions via the gas flow and to tranfer them for further acceleration into a conventional on-line isotope separator

Because of the so called plasma effect [9] its application in on-line mass separation of products from HI reactions was rather limited [10] In this case the primary beam creates a dense plasma in the recoil chamber so that the overall efficiency is considerably reduced The HI beam suppression can be achieved using different methods the most straightforward one has been applied at RIKEN by means of a high efficiency gas-filled recoil separator (GARIS) [11] an alternative one is the shadow method which we applied and will be described in section IIC

In case of light-ion-induced fission reactions the nearly isotropic angular disshytribution of the fragments makes the separation of reaction products from primary beam easier The concept of double chamber proposed at first by Taskinen et ale [12] has been applied in many laboratories and our own version will be described in section IIB

AI General layout of the ISOL facility Figure 1 gives a perspective view of the cave where the IGISOL has been

installed together with a low background detection area The isotope separator based on a 1200 angle magnet (075 m mean radius and n=12 field index) has been described in detail previously [6] it has been equipped with a new general purpose vacuum chamber which can fit with versatile ion source systems

As to thermalize high energy recoil products (in fission or HI reactions) with maximum efficiency in a small volume we have to increase the He pressure a very high speed pumping system has been installed in the basement of the cave It is composed of a set of Roots blowers (8000 rn3 h+3000 rn3 h+l000 rn3 h) and a

primary pump (120 m3 h) connected to the ion guide chamber via a 350 mm diashymeter3 m long pipe To prevent from sparks the whole system is operating at high voltage (routinely 25 kV) The second stage of the differential pumping between

3

CD SARA beam

ZnSdiaphragrn

reg Graphite diaphragm

Thrget

reg Entrance window

reg Beam supression tube

(J) Pumping line

reg 8000 m3h Roots blower

reg 3000 m 3 h Roots blower

1000 m3 h Roots blower

120 m3 h Primary purnp

Skimmer aoo acceleration

Magnet

EinzeJ lens

Counting station

) Mass separated bearn

TransIXgtrt tape

N- type Ge detector (60)

reg 1 rnrn plastic scintillator

25 m Si detector

500 Jl111 Si detector

4 cm plastic scintillator

Figure 1 Layout of the SARAIGISOL facility

4

skimmer (negatively biased) and extraction electrode (at ground potential) is evacuated with a 8000 ls oil diffusion pump

The details of the set up and control devices have been reported in a previous paper[13]

B Double chamber for fission fragments This is a modified version of a chamber designed by Taskinen et ale [12] already

mentionned The stopping volume have been increased from 8 to 20 cm3 and is fed by six cent==3 mm He ducts which ensure laminar flow pattern around the separation foil to the cent2==12 mm exit aperture (see ref [13] for details) The target chamber contains four 15 mgcm2 238U targets covered with 02 mgcm2 Al and is fed rv

by He in parallel with the stopping chamber Spontaneous or induced fission of heavy nuclei are among the most prolific mechanism to produce a great variety of neutron-rich nuclei In case of 20 MeV protons or 40 Me V alpha particles inducedshyfission of 238U the absolute cross-sections in the valley of the mass distribution are comparable to those of the thermal n-induced fission Owing to its short separation time and independence on chemical properties the IGISOL technique has proved that it was quite suitable for the study of shortlived nuclei of refractory elements from Y to Pd in the mass range A==110-120 More than twenty new isotopes have been discovered so far by the Jyvaskyla group [14] and the particular case of a collaborative work on the n-rich Ru isotopes will be addressed later

C Chamber for HI-induced fusion-evaporation reactions One of the best way to produce n-deficient nuclei consists of using HI-induced

fusion-evaporation reactions in which both target and beam particle are n-deficient In this case the concept of double chamber is no more applicable since the reacshytion products are strongly forward peaked To avoid the plasma effect we have applied the shadow method successfully used in laser spectroscopy experiments by Sprouse et ale [15] Basically the method takes advantage of the huge difference of the primary beam angular distribution and that of the evaporation residues (EVRs) after passing a thick target (about a few mgcm2) The main physical effects governing these distributions have been discussed in a recent paper [16] where it has been shown that multiscattering in the target was of major imporshytance compared to finite angular spread of projectile beam and kinematical effects due to particle evaporation after fusion The angular distributions show that for 144Sm target thickness between 1 and 3 mgcm3 the primary 40Ca beam is confined within 10 angle whereas the EVRs are spread over a 150 angle In principle a simple geometrical arrangement (beam stop or beam channel) enables to suppress more than 99 of the primary beam whereas more than 50 of EVRs can enter the recoil chamber for thermalization

5

III Spectroscopic results

AI Triaxiality in n-rich Ru isotopes In e-e nuclei the spacings observed in sequences of low-lying levels together

with the strengths of electromagnetic transitions are currently used to provide evidence of the rotation of rigid nuclei with various shapes The case of triaxial deformations is of special interest regarding this thirty years old question whether axially asymmetric nuclei are -unstable or rigid triaxial rotors

The systematics study of n-rich e-e Pd Ru Mo and Zr isotopes is of great interest to understand the gradual shape change from spherical vibrator to deformed rotor Our investigations have been focussed on the low-lying collective levels of llU1l2Ru fed by the beta decay of llO112Tc produced via charged particle-induced fission of 238 U and mass-separated using the IGI80L technique described in section II-B In addition to the discovery of the new shortlived (280 ms) 112Tc isotope we have observed in 112Ru one of the lowest second 2+ state ever reported (except in 1920S) Figure 2 shows the summary of the experimentally known low-lying collective levels in n-rich Ru nuclei obtained from our work [17] and previous data [18] Results concerning 114Ru have been obtained in a very recent experiment by measuring prompt -rays in 248Cm fission fragments with the EUROGAM large detector array [19] From this figure it can be seen that near the middle of the N=50 and 82 neutron shells the gs bands (0+2+4+) are rather similar whereas the second 2+ and first 3+ levels which belong to the -band have gradually decreasing excitation energies with a minimum occuring in 112Ru

15

shygtCIJ

--s 1~

~ CIJ c

IlJ

05 2+__ ~ -- +

middotmiddotmiddotmiddotmiddot__________2

o -------------------0+ 102R 104 l06 108 110 112 114

U R U R u Ru Ru Ru Ru

Figure 2 Some low-lying levels of 102-114Ru isotopes

6

On the basis of the two well-known empirical criteria ie the position of the 2t state below the 4t state and the sum relation on the 3t energy it is obvious that llUll2Ru and 114Ru can adopt triaxial shapes in their ground states In terms of the rigid triaxial rotor model of Davydov and Filippov [20] the values (deduced from the energies of the two 2+ levels) are 242deg 264deg and 272deg for A=110 112 and 114 respectively The agreement is also good for B(E2) ratios as was established in ref [17] and more extensively in ref [19] Microscopic lattice Hartree-Fock calculations for 1U8IlU1l2Ru [17] similar to those developed by Bonche et ale [21] have shown that the n-rich Ru are strongly defornled with a quasi degeneracy in the -direction The minima predicted in the valley are too shallow to ensure a rigid axial asymmetry as indicated by the level spacings

It becomes suggestive when comparing the level spectra of n-rich Ru nuclei with the Ba isotones in the valence nucleon scheme that it is the large neutron excess which drives the structure towards soft triaxiality

B Decay properties of very n-deficient TI isotopes Until now there are only very scarce data on the gs decay properties of the

lightest TI isotopes (A lt 184) as compared to extremely n-deficient neighbouring Pb Hg and Au nuclides Besides the evident general interest of exploring properties of isotopes close to the proton drip line there is a number of particular reasons stressing the importance of the determination of the gs decay pattern of 18UTl This nucleus can undergo many decay modes as shown in Figure 3 with estimated energies from ref [22]

First EC-delayed fission was observed and assigned to 180TI [3] precursor Second a-decay properties are poorly known for ground states and isomeric states of TI isotopes with A ~ 183 Third does large deformation as observed in this region for gs of 183185Hg strongly affect decay properties As to 180TI specifically its half-life was deduced from EC-delayed fission studies to be T 12=(97~gg)s [3] In this work the assignment was based on many cross-bombardments and by involving gs decay properties of nuclei in this region as well as systematics of HI-induced fusion-evaporation reactions However it should be corroborated by direct experiments An attempt was performed at the IRIS-faciIity using massshyseparation of products from 1 GeV protons induced spallation on the ThCx target In this experiment no a-particle which could be assigned to the decay of 180TI have been observed The high energy part of 3 spectrum (45 lt Ep lt 65 MeV) at this mass number was attributed to TI arguing that the 3-endpoint energy for the daughter 18UHg is less than 45 MeV Some 60 counts were detected and a half-life of 19plusmnO9)s was obtained for 18UTI [23] These results cannot be considered as an unambiguous identification of the isotope

7

Figure 3 Possible decay modes for the nucleus 18oTI the decay energies are derived from atomic mass evaluashy

180 tion of ref [22] 80 Hg loo

The aim of the present work was to identify 180TI and study its a-decay properties by using induced fusion-evaporation reactions and mass separation technique of the IGISOL type

In our experiment 144Sm targets enriched to 86 involving also 147Sm (4) were bombarded with 40Call+ beams accelerated by the K=90 cyclotron of the SARA facility We used 3 mgcm2 targets of 144Sm203 on 11 mgcm2 Al backshyings Bombarding energy at mid-target was chosen to be IV 210 MeV according to systematics and statistical-model expectation of the energy position for the maxishymum yield of the (p3n) evaporation channel

Typical intensities of 40Ca beams were in the range 10 to 30 pnA Shown in Figure 4 is the sum a-particle energy spectrum for the mass chain A=180 in case of the 144Sm+40Ca reaction Both the collection and counting time intervals were set to be 2 seconds in this case It is seen here that the well-known 6120 MeV a-line from 180H having a half-life of (256plusmn002)s and an a-decay branching of 35 [24] has been produced and mass-separated The width of the a-line is in agreement with the energy resolution of our SB Si detector (500 pm FWHM=37 keV at 5486 MeV) which was calibrated with standard a-sources (239pu 241 Am 244Cm) In the a-particle energy range around 575 MeV the daughter a-activity of 180Hg is present in full agreement with the a decay properties known for 176Pt(T12=63 5

bo=41 Eo=bo 5750 MeV) [25] The ratio of the peak areas of the two a-lines are in good agreement with the decay properties of the 180Hg --+ 176Pt after taking into account corrections associated with the 2 s collection and 2 s counting times

Further in Figure 4 ana-line of 607plusmn02 MeV is clearly seen the area of which represents about 30 of that of the 6120 MeV a-line of 18degHgThis is a neW a-line in the A=180 chain because it was not observed in the detailed on-line mass-separation studies performed with the 144Sm+40Ar reaction [25J There are also seen in Figure 4 weaker a-groups between 580 and 605 MeV whereas above 615 MeV only very weak a-activites are present

8

I Introduction

The study of nuclei far off the valley of stability offers the possibility to extend nuclear structure data as a function of isospin so that more stringent tests of nuclear model predictions become possible

We review our recent studies on alpha and beta decay of exotic nuclei performed with the IGISOL on-line mass separator at SARA in Grenoble The experiments using charged particle induced fission have given new information on production cross-sections [1] and properties of n-rich nuclei with A==110-130 whereas by means of HI-induced fusion evaporation reactions we have investigated two regions close to the proton drip line around A=180 and A=130

One of the physics goal of our investigations was to understand the gradual change of nuclear structure from a spherical vibrator to a deformed rotor when going trom doubly magic 132Sn to the highly deformed lOOZr In fact we have found a transition region with rather strongly deformed nuclei among the most n-rich Ru isotopes with a breaking of axial symmetry [2]

The discovery of EC 3+-delayed fission of ultra-neutron deficient isotopes in the region of Mercury-Lead [3] and the lack of data on their decay properties were mainly at the origin of our experimental programme on the TI isotopes New insight on fission barrier of cold nuclei may be gained from these investigations

The n-deficient rare-earth nuclei with A around 130 are well-known to belong to a so-called transitional region where quasi-neutrons and quasi-protons may occupy h U 2 shells From in beam studies a lot of defo~med and superdeformed bands have been observed but their connection with the low-lying levels is often an open problem Its solution can be found thanks to a precise knowledge of level schemes at low excitation energy which can be obtained from 3-decay studies [4] Our most recent experiments involving also search for proton-radioactivity will be reported

In this paper we first give a brief description of the IGISOL (Ion Guide base Isotope Separation On Line) technique and show its applications in case of two different production modes charged particle-induced fission and HI-induced fusionshyevaporation reactions

II Experimental techniques

Since 1982 the ISOL method has been applied at SARA in Grenoble for the study of nuclei produced via heavy-ion (HI)-induced fusion-evaporation reactions Initially developed with an integrated target-ion-source [5] suitable for volatile eleshyments the method has been extented to refractory elements by connecting a He-jet to a medium-current source [6] This allowed us to overcome the problem of delay time from the catcher material which was a severe limitation to the study of isoshytopes with half-lives shorter than a few tens seconds in case of non-volatile species Thus a large number of n-deficient isotopes have been identified and studied in the light rare-earth region In this technique the delay time which is approximately the

2

transit time in the capillary of the He-jet system (typically a few tens msm) cannot be avoided and moreover taking into account the maximum He flow rate compatible with the working pressure of the ion source it was not possible to go much lower than 1s range

A decisive progress was made with the IGISOL technique pioneered by the Jyvaskyla group [7] in which the delay time between production and detection afshyter mass separation is mainly due to the mean evacuation time of the He-pressurized recoil chamber This group has shown that in the case of light ion-induced fusionshyevaporation reactions the overall transmission efficiency can be of the order of 1 even for isotopes with short half-lives down to 100 JLS [8] In addition the IGISOL technique has very stable operating conditions and its chemical non-selectivity makes it very attractive for refractory elements Its basic principle takes advantage of the fact that nuclear reaction products recoiling out of a target and thermalized in He may survive as singly charged ions for about 10 ms This time is usually long enough to extract the ions via the gas flow and to tranfer them for further acceleration into a conventional on-line isotope separator

Because of the so called plasma effect [9] its application in on-line mass separation of products from HI reactions was rather limited [10] In this case the primary beam creates a dense plasma in the recoil chamber so that the overall efficiency is considerably reduced The HI beam suppression can be achieved using different methods the most straightforward one has been applied at RIKEN by means of a high efficiency gas-filled recoil separator (GARIS) [11] an alternative one is the shadow method which we applied and will be described in section IIC

In case of light-ion-induced fission reactions the nearly isotropic angular disshytribution of the fragments makes the separation of reaction products from primary beam easier The concept of double chamber proposed at first by Taskinen et ale [12] has been applied in many laboratories and our own version will be described in section IIB

AI General layout of the ISOL facility Figure 1 gives a perspective view of the cave where the IGISOL has been

installed together with a low background detection area The isotope separator based on a 1200 angle magnet (075 m mean radius and n=12 field index) has been described in detail previously [6] it has been equipped with a new general purpose vacuum chamber which can fit with versatile ion source systems

As to thermalize high energy recoil products (in fission or HI reactions) with maximum efficiency in a small volume we have to increase the He pressure a very high speed pumping system has been installed in the basement of the cave It is composed of a set of Roots blowers (8000 rn3 h+3000 rn3 h+l000 rn3 h) and a

primary pump (120 m3 h) connected to the ion guide chamber via a 350 mm diashymeter3 m long pipe To prevent from sparks the whole system is operating at high voltage (routinely 25 kV) The second stage of the differential pumping between

3

CD SARA beam

ZnSdiaphragrn

reg Graphite diaphragm

Thrget

reg Entrance window

reg Beam supression tube

(J) Pumping line

reg 8000 m3h Roots blower

reg 3000 m 3 h Roots blower

1000 m3 h Roots blower

120 m3 h Primary purnp

Skimmer aoo acceleration

Magnet

EinzeJ lens

Counting station

) Mass separated bearn

TransIXgtrt tape

N- type Ge detector (60)

reg 1 rnrn plastic scintillator

25 m Si detector

500 Jl111 Si detector

4 cm plastic scintillator

Figure 1 Layout of the SARAIGISOL facility

4

skimmer (negatively biased) and extraction electrode (at ground potential) is evacuated with a 8000 ls oil diffusion pump

The details of the set up and control devices have been reported in a previous paper[13]

B Double chamber for fission fragments This is a modified version of a chamber designed by Taskinen et ale [12] already

mentionned The stopping volume have been increased from 8 to 20 cm3 and is fed by six cent==3 mm He ducts which ensure laminar flow pattern around the separation foil to the cent2==12 mm exit aperture (see ref [13] for details) The target chamber contains four 15 mgcm2 238U targets covered with 02 mgcm2 Al and is fed rv

by He in parallel with the stopping chamber Spontaneous or induced fission of heavy nuclei are among the most prolific mechanism to produce a great variety of neutron-rich nuclei In case of 20 MeV protons or 40 Me V alpha particles inducedshyfission of 238U the absolute cross-sections in the valley of the mass distribution are comparable to those of the thermal n-induced fission Owing to its short separation time and independence on chemical properties the IGISOL technique has proved that it was quite suitable for the study of shortlived nuclei of refractory elements from Y to Pd in the mass range A==110-120 More than twenty new isotopes have been discovered so far by the Jyvaskyla group [14] and the particular case of a collaborative work on the n-rich Ru isotopes will be addressed later

C Chamber for HI-induced fusion-evaporation reactions One of the best way to produce n-deficient nuclei consists of using HI-induced

fusion-evaporation reactions in which both target and beam particle are n-deficient In this case the concept of double chamber is no more applicable since the reacshytion products are strongly forward peaked To avoid the plasma effect we have applied the shadow method successfully used in laser spectroscopy experiments by Sprouse et ale [15] Basically the method takes advantage of the huge difference of the primary beam angular distribution and that of the evaporation residues (EVRs) after passing a thick target (about a few mgcm2) The main physical effects governing these distributions have been discussed in a recent paper [16] where it has been shown that multiscattering in the target was of major imporshytance compared to finite angular spread of projectile beam and kinematical effects due to particle evaporation after fusion The angular distributions show that for 144Sm target thickness between 1 and 3 mgcm3 the primary 40Ca beam is confined within 10 angle whereas the EVRs are spread over a 150 angle In principle a simple geometrical arrangement (beam stop or beam channel) enables to suppress more than 99 of the primary beam whereas more than 50 of EVRs can enter the recoil chamber for thermalization

5

III Spectroscopic results

AI Triaxiality in n-rich Ru isotopes In e-e nuclei the spacings observed in sequences of low-lying levels together

with the strengths of electromagnetic transitions are currently used to provide evidence of the rotation of rigid nuclei with various shapes The case of triaxial deformations is of special interest regarding this thirty years old question whether axially asymmetric nuclei are -unstable or rigid triaxial rotors

The systematics study of n-rich e-e Pd Ru Mo and Zr isotopes is of great interest to understand the gradual shape change from spherical vibrator to deformed rotor Our investigations have been focussed on the low-lying collective levels of llU1l2Ru fed by the beta decay of llO112Tc produced via charged particle-induced fission of 238 U and mass-separated using the IGI80L technique described in section II-B In addition to the discovery of the new shortlived (280 ms) 112Tc isotope we have observed in 112Ru one of the lowest second 2+ state ever reported (except in 1920S) Figure 2 shows the summary of the experimentally known low-lying collective levels in n-rich Ru nuclei obtained from our work [17] and previous data [18] Results concerning 114Ru have been obtained in a very recent experiment by measuring prompt -rays in 248Cm fission fragments with the EUROGAM large detector array [19] From this figure it can be seen that near the middle of the N=50 and 82 neutron shells the gs bands (0+2+4+) are rather similar whereas the second 2+ and first 3+ levels which belong to the -band have gradually decreasing excitation energies with a minimum occuring in 112Ru

15

shygtCIJ

--s 1~

~ CIJ c

IlJ

05 2+__ ~ -- +

middotmiddotmiddotmiddotmiddot__________2

o -------------------0+ 102R 104 l06 108 110 112 114

U R U R u Ru Ru Ru Ru

Figure 2 Some low-lying levels of 102-114Ru isotopes

6

On the basis of the two well-known empirical criteria ie the position of the 2t state below the 4t state and the sum relation on the 3t energy it is obvious that llUll2Ru and 114Ru can adopt triaxial shapes in their ground states In terms of the rigid triaxial rotor model of Davydov and Filippov [20] the values (deduced from the energies of the two 2+ levels) are 242deg 264deg and 272deg for A=110 112 and 114 respectively The agreement is also good for B(E2) ratios as was established in ref [17] and more extensively in ref [19] Microscopic lattice Hartree-Fock calculations for 1U8IlU1l2Ru [17] similar to those developed by Bonche et ale [21] have shown that the n-rich Ru are strongly defornled with a quasi degeneracy in the -direction The minima predicted in the valley are too shallow to ensure a rigid axial asymmetry as indicated by the level spacings

It becomes suggestive when comparing the level spectra of n-rich Ru nuclei with the Ba isotones in the valence nucleon scheme that it is the large neutron excess which drives the structure towards soft triaxiality

B Decay properties of very n-deficient TI isotopes Until now there are only very scarce data on the gs decay properties of the

lightest TI isotopes (A lt 184) as compared to extremely n-deficient neighbouring Pb Hg and Au nuclides Besides the evident general interest of exploring properties of isotopes close to the proton drip line there is a number of particular reasons stressing the importance of the determination of the gs decay pattern of 18UTl This nucleus can undergo many decay modes as shown in Figure 3 with estimated energies from ref [22]

First EC-delayed fission was observed and assigned to 180TI [3] precursor Second a-decay properties are poorly known for ground states and isomeric states of TI isotopes with A ~ 183 Third does large deformation as observed in this region for gs of 183185Hg strongly affect decay properties As to 180TI specifically its half-life was deduced from EC-delayed fission studies to be T 12=(97~gg)s [3] In this work the assignment was based on many cross-bombardments and by involving gs decay properties of nuclei in this region as well as systematics of HI-induced fusion-evaporation reactions However it should be corroborated by direct experiments An attempt was performed at the IRIS-faciIity using massshyseparation of products from 1 GeV protons induced spallation on the ThCx target In this experiment no a-particle which could be assigned to the decay of 180TI have been observed The high energy part of 3 spectrum (45 lt Ep lt 65 MeV) at this mass number was attributed to TI arguing that the 3-endpoint energy for the daughter 18UHg is less than 45 MeV Some 60 counts were detected and a half-life of 19plusmnO9)s was obtained for 18UTI [23] These results cannot be considered as an unambiguous identification of the isotope

7

Figure 3 Possible decay modes for the nucleus 18oTI the decay energies are derived from atomic mass evaluashy

180 tion of ref [22] 80 Hg loo

The aim of the present work was to identify 180TI and study its a-decay properties by using induced fusion-evaporation reactions and mass separation technique of the IGISOL type

In our experiment 144Sm targets enriched to 86 involving also 147Sm (4) were bombarded with 40Call+ beams accelerated by the K=90 cyclotron of the SARA facility We used 3 mgcm2 targets of 144Sm203 on 11 mgcm2 Al backshyings Bombarding energy at mid-target was chosen to be IV 210 MeV according to systematics and statistical-model expectation of the energy position for the maxishymum yield of the (p3n) evaporation channel

Typical intensities of 40Ca beams were in the range 10 to 30 pnA Shown in Figure 4 is the sum a-particle energy spectrum for the mass chain A=180 in case of the 144Sm+40Ca reaction Both the collection and counting time intervals were set to be 2 seconds in this case It is seen here that the well-known 6120 MeV a-line from 180H having a half-life of (256plusmn002)s and an a-decay branching of 35 [24] has been produced and mass-separated The width of the a-line is in agreement with the energy resolution of our SB Si detector (500 pm FWHM=37 keV at 5486 MeV) which was calibrated with standard a-sources (239pu 241 Am 244Cm) In the a-particle energy range around 575 MeV the daughter a-activity of 180Hg is present in full agreement with the a decay properties known for 176Pt(T12=63 5

bo=41 Eo=bo 5750 MeV) [25] The ratio of the peak areas of the two a-lines are in good agreement with the decay properties of the 180Hg --+ 176Pt after taking into account corrections associated with the 2 s collection and 2 s counting times

Further in Figure 4 ana-line of 607plusmn02 MeV is clearly seen the area of which represents about 30 of that of the 6120 MeV a-line of 18degHgThis is a neW a-line in the A=180 chain because it was not observed in the detailed on-line mass-separation studies performed with the 144Sm+40Ar reaction [25J There are also seen in Figure 4 weaker a-groups between 580 and 605 MeV whereas above 615 MeV only very weak a-activites are present

8

transit time in the capillary of the He-jet system (typically a few tens msm) cannot be avoided and moreover taking into account the maximum He flow rate compatible with the working pressure of the ion source it was not possible to go much lower than 1s range

A decisive progress was made with the IGISOL technique pioneered by the Jyvaskyla group [7] in which the delay time between production and detection afshyter mass separation is mainly due to the mean evacuation time of the He-pressurized recoil chamber This group has shown that in the case of light ion-induced fusionshyevaporation reactions the overall transmission efficiency can be of the order of 1 even for isotopes with short half-lives down to 100 JLS [8] In addition the IGISOL technique has very stable operating conditions and its chemical non-selectivity makes it very attractive for refractory elements Its basic principle takes advantage of the fact that nuclear reaction products recoiling out of a target and thermalized in He may survive as singly charged ions for about 10 ms This time is usually long enough to extract the ions via the gas flow and to tranfer them for further acceleration into a conventional on-line isotope separator

Because of the so called plasma effect [9] its application in on-line mass separation of products from HI reactions was rather limited [10] In this case the primary beam creates a dense plasma in the recoil chamber so that the overall efficiency is considerably reduced The HI beam suppression can be achieved using different methods the most straightforward one has been applied at RIKEN by means of a high efficiency gas-filled recoil separator (GARIS) [11] an alternative one is the shadow method which we applied and will be described in section IIC

In case of light-ion-induced fission reactions the nearly isotropic angular disshytribution of the fragments makes the separation of reaction products from primary beam easier The concept of double chamber proposed at first by Taskinen et ale [12] has been applied in many laboratories and our own version will be described in section IIB

AI General layout of the ISOL facility Figure 1 gives a perspective view of the cave where the IGISOL has been

installed together with a low background detection area The isotope separator based on a 1200 angle magnet (075 m mean radius and n=12 field index) has been described in detail previously [6] it has been equipped with a new general purpose vacuum chamber which can fit with versatile ion source systems

As to thermalize high energy recoil products (in fission or HI reactions) with maximum efficiency in a small volume we have to increase the He pressure a very high speed pumping system has been installed in the basement of the cave It is composed of a set of Roots blowers (8000 rn3 h+3000 rn3 h+l000 rn3 h) and a

primary pump (120 m3 h) connected to the ion guide chamber via a 350 mm diashymeter3 m long pipe To prevent from sparks the whole system is operating at high voltage (routinely 25 kV) The second stage of the differential pumping between

3

CD SARA beam

ZnSdiaphragrn

reg Graphite diaphragm

Thrget

reg Entrance window

reg Beam supression tube

(J) Pumping line

reg 8000 m3h Roots blower

reg 3000 m 3 h Roots blower

1000 m3 h Roots blower

120 m3 h Primary purnp

Skimmer aoo acceleration

Magnet

EinzeJ lens

Counting station

) Mass separated bearn

TransIXgtrt tape

N- type Ge detector (60)

reg 1 rnrn plastic scintillator

25 m Si detector

500 Jl111 Si detector

4 cm plastic scintillator

Figure 1 Layout of the SARAIGISOL facility

4

skimmer (negatively biased) and extraction electrode (at ground potential) is evacuated with a 8000 ls oil diffusion pump

The details of the set up and control devices have been reported in a previous paper[13]

B Double chamber for fission fragments This is a modified version of a chamber designed by Taskinen et ale [12] already

mentionned The stopping volume have been increased from 8 to 20 cm3 and is fed by six cent==3 mm He ducts which ensure laminar flow pattern around the separation foil to the cent2==12 mm exit aperture (see ref [13] for details) The target chamber contains four 15 mgcm2 238U targets covered with 02 mgcm2 Al and is fed rv

by He in parallel with the stopping chamber Spontaneous or induced fission of heavy nuclei are among the most prolific mechanism to produce a great variety of neutron-rich nuclei In case of 20 MeV protons or 40 Me V alpha particles inducedshyfission of 238U the absolute cross-sections in the valley of the mass distribution are comparable to those of the thermal n-induced fission Owing to its short separation time and independence on chemical properties the IGISOL technique has proved that it was quite suitable for the study of shortlived nuclei of refractory elements from Y to Pd in the mass range A==110-120 More than twenty new isotopes have been discovered so far by the Jyvaskyla group [14] and the particular case of a collaborative work on the n-rich Ru isotopes will be addressed later

C Chamber for HI-induced fusion-evaporation reactions One of the best way to produce n-deficient nuclei consists of using HI-induced

fusion-evaporation reactions in which both target and beam particle are n-deficient In this case the concept of double chamber is no more applicable since the reacshytion products are strongly forward peaked To avoid the plasma effect we have applied the shadow method successfully used in laser spectroscopy experiments by Sprouse et ale [15] Basically the method takes advantage of the huge difference of the primary beam angular distribution and that of the evaporation residues (EVRs) after passing a thick target (about a few mgcm2) The main physical effects governing these distributions have been discussed in a recent paper [16] where it has been shown that multiscattering in the target was of major imporshytance compared to finite angular spread of projectile beam and kinematical effects due to particle evaporation after fusion The angular distributions show that for 144Sm target thickness between 1 and 3 mgcm3 the primary 40Ca beam is confined within 10 angle whereas the EVRs are spread over a 150 angle In principle a simple geometrical arrangement (beam stop or beam channel) enables to suppress more than 99 of the primary beam whereas more than 50 of EVRs can enter the recoil chamber for thermalization

5

III Spectroscopic results

AI Triaxiality in n-rich Ru isotopes In e-e nuclei the spacings observed in sequences of low-lying levels together

with the strengths of electromagnetic transitions are currently used to provide evidence of the rotation of rigid nuclei with various shapes The case of triaxial deformations is of special interest regarding this thirty years old question whether axially asymmetric nuclei are -unstable or rigid triaxial rotors

The systematics study of n-rich e-e Pd Ru Mo and Zr isotopes is of great interest to understand the gradual shape change from spherical vibrator to deformed rotor Our investigations have been focussed on the low-lying collective levels of llU1l2Ru fed by the beta decay of llO112Tc produced via charged particle-induced fission of 238 U and mass-separated using the IGI80L technique described in section II-B In addition to the discovery of the new shortlived (280 ms) 112Tc isotope we have observed in 112Ru one of the lowest second 2+ state ever reported (except in 1920S) Figure 2 shows the summary of the experimentally known low-lying collective levels in n-rich Ru nuclei obtained from our work [17] and previous data [18] Results concerning 114Ru have been obtained in a very recent experiment by measuring prompt -rays in 248Cm fission fragments with the EUROGAM large detector array [19] From this figure it can be seen that near the middle of the N=50 and 82 neutron shells the gs bands (0+2+4+) are rather similar whereas the second 2+ and first 3+ levels which belong to the -band have gradually decreasing excitation energies with a minimum occuring in 112Ru

15

shygtCIJ

--s 1~

~ CIJ c

IlJ

05 2+__ ~ -- +

middotmiddotmiddotmiddotmiddot__________2

o -------------------0+ 102R 104 l06 108 110 112 114

U R U R u Ru Ru Ru Ru

Figure 2 Some low-lying levels of 102-114Ru isotopes

6

On the basis of the two well-known empirical criteria ie the position of the 2t state below the 4t state and the sum relation on the 3t energy it is obvious that llUll2Ru and 114Ru can adopt triaxial shapes in their ground states In terms of the rigid triaxial rotor model of Davydov and Filippov [20] the values (deduced from the energies of the two 2+ levels) are 242deg 264deg and 272deg for A=110 112 and 114 respectively The agreement is also good for B(E2) ratios as was established in ref [17] and more extensively in ref [19] Microscopic lattice Hartree-Fock calculations for 1U8IlU1l2Ru [17] similar to those developed by Bonche et ale [21] have shown that the n-rich Ru are strongly defornled with a quasi degeneracy in the -direction The minima predicted in the valley are too shallow to ensure a rigid axial asymmetry as indicated by the level spacings

It becomes suggestive when comparing the level spectra of n-rich Ru nuclei with the Ba isotones in the valence nucleon scheme that it is the large neutron excess which drives the structure towards soft triaxiality

B Decay properties of very n-deficient TI isotopes Until now there are only very scarce data on the gs decay properties of the

lightest TI isotopes (A lt 184) as compared to extremely n-deficient neighbouring Pb Hg and Au nuclides Besides the evident general interest of exploring properties of isotopes close to the proton drip line there is a number of particular reasons stressing the importance of the determination of the gs decay pattern of 18UTl This nucleus can undergo many decay modes as shown in Figure 3 with estimated energies from ref [22]

First EC-delayed fission was observed and assigned to 180TI [3] precursor Second a-decay properties are poorly known for ground states and isomeric states of TI isotopes with A ~ 183 Third does large deformation as observed in this region for gs of 183185Hg strongly affect decay properties As to 180TI specifically its half-life was deduced from EC-delayed fission studies to be T 12=(97~gg)s [3] In this work the assignment was based on many cross-bombardments and by involving gs decay properties of nuclei in this region as well as systematics of HI-induced fusion-evaporation reactions However it should be corroborated by direct experiments An attempt was performed at the IRIS-faciIity using massshyseparation of products from 1 GeV protons induced spallation on the ThCx target In this experiment no a-particle which could be assigned to the decay of 180TI have been observed The high energy part of 3 spectrum (45 lt Ep lt 65 MeV) at this mass number was attributed to TI arguing that the 3-endpoint energy for the daughter 18UHg is less than 45 MeV Some 60 counts were detected and a half-life of 19plusmnO9)s was obtained for 18UTI [23] These results cannot be considered as an unambiguous identification of the isotope

7

Figure 3 Possible decay modes for the nucleus 18oTI the decay energies are derived from atomic mass evaluashy

180 tion of ref [22] 80 Hg loo

The aim of the present work was to identify 180TI and study its a-decay properties by using induced fusion-evaporation reactions and mass separation technique of the IGISOL type

In our experiment 144Sm targets enriched to 86 involving also 147Sm (4) were bombarded with 40Call+ beams accelerated by the K=90 cyclotron of the SARA facility We used 3 mgcm2 targets of 144Sm203 on 11 mgcm2 Al backshyings Bombarding energy at mid-target was chosen to be IV 210 MeV according to systematics and statistical-model expectation of the energy position for the maxishymum yield of the (p3n) evaporation channel

Typical intensities of 40Ca beams were in the range 10 to 30 pnA Shown in Figure 4 is the sum a-particle energy spectrum for the mass chain A=180 in case of the 144Sm+40Ca reaction Both the collection and counting time intervals were set to be 2 seconds in this case It is seen here that the well-known 6120 MeV a-line from 180H having a half-life of (256plusmn002)s and an a-decay branching of 35 [24] has been produced and mass-separated The width of the a-line is in agreement with the energy resolution of our SB Si detector (500 pm FWHM=37 keV at 5486 MeV) which was calibrated with standard a-sources (239pu 241 Am 244Cm) In the a-particle energy range around 575 MeV the daughter a-activity of 180Hg is present in full agreement with the a decay properties known for 176Pt(T12=63 5

bo=41 Eo=bo 5750 MeV) [25] The ratio of the peak areas of the two a-lines are in good agreement with the decay properties of the 180Hg --+ 176Pt after taking into account corrections associated with the 2 s collection and 2 s counting times

Further in Figure 4 ana-line of 607plusmn02 MeV is clearly seen the area of which represents about 30 of that of the 6120 MeV a-line of 18degHgThis is a neW a-line in the A=180 chain because it was not observed in the detailed on-line mass-separation studies performed with the 144Sm+40Ar reaction [25J There are also seen in Figure 4 weaker a-groups between 580 and 605 MeV whereas above 615 MeV only very weak a-activites are present

8

CD SARA beam

ZnSdiaphragrn

reg Graphite diaphragm

Thrget

reg Entrance window

reg Beam supression tube

(J) Pumping line

reg 8000 m3h Roots blower

reg 3000 m 3 h Roots blower

1000 m3 h Roots blower

120 m3 h Primary purnp

Skimmer aoo acceleration

Magnet

EinzeJ lens

Counting station

) Mass separated bearn

TransIXgtrt tape

N- type Ge detector (60)

reg 1 rnrn plastic scintillator

25 m Si detector

500 Jl111 Si detector

4 cm plastic scintillator

Figure 1 Layout of the SARAIGISOL facility

4

skimmer (negatively biased) and extraction electrode (at ground potential) is evacuated with a 8000 ls oil diffusion pump

The details of the set up and control devices have been reported in a previous paper[13]

B Double chamber for fission fragments This is a modified version of a chamber designed by Taskinen et ale [12] already

mentionned The stopping volume have been increased from 8 to 20 cm3 and is fed by six cent==3 mm He ducts which ensure laminar flow pattern around the separation foil to the cent2==12 mm exit aperture (see ref [13] for details) The target chamber contains four 15 mgcm2 238U targets covered with 02 mgcm2 Al and is fed rv

by He in parallel with the stopping chamber Spontaneous or induced fission of heavy nuclei are among the most prolific mechanism to produce a great variety of neutron-rich nuclei In case of 20 MeV protons or 40 Me V alpha particles inducedshyfission of 238U the absolute cross-sections in the valley of the mass distribution are comparable to those of the thermal n-induced fission Owing to its short separation time and independence on chemical properties the IGISOL technique has proved that it was quite suitable for the study of shortlived nuclei of refractory elements from Y to Pd in the mass range A==110-120 More than twenty new isotopes have been discovered so far by the Jyvaskyla group [14] and the particular case of a collaborative work on the n-rich Ru isotopes will be addressed later

C Chamber for HI-induced fusion-evaporation reactions One of the best way to produce n-deficient nuclei consists of using HI-induced

fusion-evaporation reactions in which both target and beam particle are n-deficient In this case the concept of double chamber is no more applicable since the reacshytion products are strongly forward peaked To avoid the plasma effect we have applied the shadow method successfully used in laser spectroscopy experiments by Sprouse et ale [15] Basically the method takes advantage of the huge difference of the primary beam angular distribution and that of the evaporation residues (EVRs) after passing a thick target (about a few mgcm2) The main physical effects governing these distributions have been discussed in a recent paper [16] where it has been shown that multiscattering in the target was of major imporshytance compared to finite angular spread of projectile beam and kinematical effects due to particle evaporation after fusion The angular distributions show that for 144Sm target thickness between 1 and 3 mgcm3 the primary 40Ca beam is confined within 10 angle whereas the EVRs are spread over a 150 angle In principle a simple geometrical arrangement (beam stop or beam channel) enables to suppress more than 99 of the primary beam whereas more than 50 of EVRs can enter the recoil chamber for thermalization

5

III Spectroscopic results

AI Triaxiality in n-rich Ru isotopes In e-e nuclei the spacings observed in sequences of low-lying levels together

with the strengths of electromagnetic transitions are currently used to provide evidence of the rotation of rigid nuclei with various shapes The case of triaxial deformations is of special interest regarding this thirty years old question whether axially asymmetric nuclei are -unstable or rigid triaxial rotors

The systematics study of n-rich e-e Pd Ru Mo and Zr isotopes is of great interest to understand the gradual shape change from spherical vibrator to deformed rotor Our investigations have been focussed on the low-lying collective levels of llU1l2Ru fed by the beta decay of llO112Tc produced via charged particle-induced fission of 238 U and mass-separated using the IGI80L technique described in section II-B In addition to the discovery of the new shortlived (280 ms) 112Tc isotope we have observed in 112Ru one of the lowest second 2+ state ever reported (except in 1920S) Figure 2 shows the summary of the experimentally known low-lying collective levels in n-rich Ru nuclei obtained from our work [17] and previous data [18] Results concerning 114Ru have been obtained in a very recent experiment by measuring prompt -rays in 248Cm fission fragments with the EUROGAM large detector array [19] From this figure it can be seen that near the middle of the N=50 and 82 neutron shells the gs bands (0+2+4+) are rather similar whereas the second 2+ and first 3+ levels which belong to the -band have gradually decreasing excitation energies with a minimum occuring in 112Ru

15

shygtCIJ

--s 1~

~ CIJ c

IlJ

05 2+__ ~ -- +

middotmiddotmiddotmiddotmiddot__________2

o -------------------0+ 102R 104 l06 108 110 112 114

U R U R u Ru Ru Ru Ru

Figure 2 Some low-lying levels of 102-114Ru isotopes

6

On the basis of the two well-known empirical criteria ie the position of the 2t state below the 4t state and the sum relation on the 3t energy it is obvious that llUll2Ru and 114Ru can adopt triaxial shapes in their ground states In terms of the rigid triaxial rotor model of Davydov and Filippov [20] the values (deduced from the energies of the two 2+ levels) are 242deg 264deg and 272deg for A=110 112 and 114 respectively The agreement is also good for B(E2) ratios as was established in ref [17] and more extensively in ref [19] Microscopic lattice Hartree-Fock calculations for 1U8IlU1l2Ru [17] similar to those developed by Bonche et ale [21] have shown that the n-rich Ru are strongly defornled with a quasi degeneracy in the -direction The minima predicted in the valley are too shallow to ensure a rigid axial asymmetry as indicated by the level spacings

It becomes suggestive when comparing the level spectra of n-rich Ru nuclei with the Ba isotones in the valence nucleon scheme that it is the large neutron excess which drives the structure towards soft triaxiality

B Decay properties of very n-deficient TI isotopes Until now there are only very scarce data on the gs decay properties of the

lightest TI isotopes (A lt 184) as compared to extremely n-deficient neighbouring Pb Hg and Au nuclides Besides the evident general interest of exploring properties of isotopes close to the proton drip line there is a number of particular reasons stressing the importance of the determination of the gs decay pattern of 18UTl This nucleus can undergo many decay modes as shown in Figure 3 with estimated energies from ref [22]

First EC-delayed fission was observed and assigned to 180TI [3] precursor Second a-decay properties are poorly known for ground states and isomeric states of TI isotopes with A ~ 183 Third does large deformation as observed in this region for gs of 183185Hg strongly affect decay properties As to 180TI specifically its half-life was deduced from EC-delayed fission studies to be T 12=(97~gg)s [3] In this work the assignment was based on many cross-bombardments and by involving gs decay properties of nuclei in this region as well as systematics of HI-induced fusion-evaporation reactions However it should be corroborated by direct experiments An attempt was performed at the IRIS-faciIity using massshyseparation of products from 1 GeV protons induced spallation on the ThCx target In this experiment no a-particle which could be assigned to the decay of 180TI have been observed The high energy part of 3 spectrum (45 lt Ep lt 65 MeV) at this mass number was attributed to TI arguing that the 3-endpoint energy for the daughter 18UHg is less than 45 MeV Some 60 counts were detected and a half-life of 19plusmnO9)s was obtained for 18UTI [23] These results cannot be considered as an unambiguous identification of the isotope

7

Figure 3 Possible decay modes for the nucleus 18oTI the decay energies are derived from atomic mass evaluashy

180 tion of ref [22] 80 Hg loo

The aim of the present work was to identify 180TI and study its a-decay properties by using induced fusion-evaporation reactions and mass separation technique of the IGISOL type

In our experiment 144Sm targets enriched to 86 involving also 147Sm (4) were bombarded with 40Call+ beams accelerated by the K=90 cyclotron of the SARA facility We used 3 mgcm2 targets of 144Sm203 on 11 mgcm2 Al backshyings Bombarding energy at mid-target was chosen to be IV 210 MeV according to systematics and statistical-model expectation of the energy position for the maxishymum yield of the (p3n) evaporation channel

Typical intensities of 40Ca beams were in the range 10 to 30 pnA Shown in Figure 4 is the sum a-particle energy spectrum for the mass chain A=180 in case of the 144Sm+40Ca reaction Both the collection and counting time intervals were set to be 2 seconds in this case It is seen here that the well-known 6120 MeV a-line from 180H having a half-life of (256plusmn002)s and an a-decay branching of 35 [24] has been produced and mass-separated The width of the a-line is in agreement with the energy resolution of our SB Si detector (500 pm FWHM=37 keV at 5486 MeV) which was calibrated with standard a-sources (239pu 241 Am 244Cm) In the a-particle energy range around 575 MeV the daughter a-activity of 180Hg is present in full agreement with the a decay properties known for 176Pt(T12=63 5

bo=41 Eo=bo 5750 MeV) [25] The ratio of the peak areas of the two a-lines are in good agreement with the decay properties of the 180Hg --+ 176Pt after taking into account corrections associated with the 2 s collection and 2 s counting times

Further in Figure 4 ana-line of 607plusmn02 MeV is clearly seen the area of which represents about 30 of that of the 6120 MeV a-line of 18degHgThis is a neW a-line in the A=180 chain because it was not observed in the detailed on-line mass-separation studies performed with the 144Sm+40Ar reaction [25J There are also seen in Figure 4 weaker a-groups between 580 and 605 MeV whereas above 615 MeV only very weak a-activites are present

8

skimmer (negatively biased) and extraction electrode (at ground potential) is evacuated with a 8000 ls oil diffusion pump

The details of the set up and control devices have been reported in a previous paper[13]

B Double chamber for fission fragments This is a modified version of a chamber designed by Taskinen et ale [12] already

mentionned The stopping volume have been increased from 8 to 20 cm3 and is fed by six cent==3 mm He ducts which ensure laminar flow pattern around the separation foil to the cent2==12 mm exit aperture (see ref [13] for details) The target chamber contains four 15 mgcm2 238U targets covered with 02 mgcm2 Al and is fed rv

by He in parallel with the stopping chamber Spontaneous or induced fission of heavy nuclei are among the most prolific mechanism to produce a great variety of neutron-rich nuclei In case of 20 MeV protons or 40 Me V alpha particles inducedshyfission of 238U the absolute cross-sections in the valley of the mass distribution are comparable to those of the thermal n-induced fission Owing to its short separation time and independence on chemical properties the IGISOL technique has proved that it was quite suitable for the study of shortlived nuclei of refractory elements from Y to Pd in the mass range A==110-120 More than twenty new isotopes have been discovered so far by the Jyvaskyla group [14] and the particular case of a collaborative work on the n-rich Ru isotopes will be addressed later

C Chamber for HI-induced fusion-evaporation reactions One of the best way to produce n-deficient nuclei consists of using HI-induced

fusion-evaporation reactions in which both target and beam particle are n-deficient In this case the concept of double chamber is no more applicable since the reacshytion products are strongly forward peaked To avoid the plasma effect we have applied the shadow method successfully used in laser spectroscopy experiments by Sprouse et ale [15] Basically the method takes advantage of the huge difference of the primary beam angular distribution and that of the evaporation residues (EVRs) after passing a thick target (about a few mgcm2) The main physical effects governing these distributions have been discussed in a recent paper [16] where it has been shown that multiscattering in the target was of major imporshytance compared to finite angular spread of projectile beam and kinematical effects due to particle evaporation after fusion The angular distributions show that for 144Sm target thickness between 1 and 3 mgcm3 the primary 40Ca beam is confined within 10 angle whereas the EVRs are spread over a 150 angle In principle a simple geometrical arrangement (beam stop or beam channel) enables to suppress more than 99 of the primary beam whereas more than 50 of EVRs can enter the recoil chamber for thermalization

5

III Spectroscopic results

AI Triaxiality in n-rich Ru isotopes In e-e nuclei the spacings observed in sequences of low-lying levels together

with the strengths of electromagnetic transitions are currently used to provide evidence of the rotation of rigid nuclei with various shapes The case of triaxial deformations is of special interest regarding this thirty years old question whether axially asymmetric nuclei are -unstable or rigid triaxial rotors

The systematics study of n-rich e-e Pd Ru Mo and Zr isotopes is of great interest to understand the gradual shape change from spherical vibrator to deformed rotor Our investigations have been focussed on the low-lying collective levels of llU1l2Ru fed by the beta decay of llO112Tc produced via charged particle-induced fission of 238 U and mass-separated using the IGI80L technique described in section II-B In addition to the discovery of the new shortlived (280 ms) 112Tc isotope we have observed in 112Ru one of the lowest second 2+ state ever reported (except in 1920S) Figure 2 shows the summary of the experimentally known low-lying collective levels in n-rich Ru nuclei obtained from our work [17] and previous data [18] Results concerning 114Ru have been obtained in a very recent experiment by measuring prompt -rays in 248Cm fission fragments with the EUROGAM large detector array [19] From this figure it can be seen that near the middle of the N=50 and 82 neutron shells the gs bands (0+2+4+) are rather similar whereas the second 2+ and first 3+ levels which belong to the -band have gradually decreasing excitation energies with a minimum occuring in 112Ru

15

shygtCIJ

--s 1~

~ CIJ c

IlJ

05 2+__ ~ -- +

middotmiddotmiddotmiddotmiddot__________2

o -------------------0+ 102R 104 l06 108 110 112 114

U R U R u Ru Ru Ru Ru

Figure 2 Some low-lying levels of 102-114Ru isotopes

6

On the basis of the two well-known empirical criteria ie the position of the 2t state below the 4t state and the sum relation on the 3t energy it is obvious that llUll2Ru and 114Ru can adopt triaxial shapes in their ground states In terms of the rigid triaxial rotor model of Davydov and Filippov [20] the values (deduced from the energies of the two 2+ levels) are 242deg 264deg and 272deg for A=110 112 and 114 respectively The agreement is also good for B(E2) ratios as was established in ref [17] and more extensively in ref [19] Microscopic lattice Hartree-Fock calculations for 1U8IlU1l2Ru [17] similar to those developed by Bonche et ale [21] have shown that the n-rich Ru are strongly defornled with a quasi degeneracy in the -direction The minima predicted in the valley are too shallow to ensure a rigid axial asymmetry as indicated by the level spacings

It becomes suggestive when comparing the level spectra of n-rich Ru nuclei with the Ba isotones in the valence nucleon scheme that it is the large neutron excess which drives the structure towards soft triaxiality

B Decay properties of very n-deficient TI isotopes Until now there are only very scarce data on the gs decay properties of the

lightest TI isotopes (A lt 184) as compared to extremely n-deficient neighbouring Pb Hg and Au nuclides Besides the evident general interest of exploring properties of isotopes close to the proton drip line there is a number of particular reasons stressing the importance of the determination of the gs decay pattern of 18UTl This nucleus can undergo many decay modes as shown in Figure 3 with estimated energies from ref [22]

First EC-delayed fission was observed and assigned to 180TI [3] precursor Second a-decay properties are poorly known for ground states and isomeric states of TI isotopes with A ~ 183 Third does large deformation as observed in this region for gs of 183185Hg strongly affect decay properties As to 180TI specifically its half-life was deduced from EC-delayed fission studies to be T 12=(97~gg)s [3] In this work the assignment was based on many cross-bombardments and by involving gs decay properties of nuclei in this region as well as systematics of HI-induced fusion-evaporation reactions However it should be corroborated by direct experiments An attempt was performed at the IRIS-faciIity using massshyseparation of products from 1 GeV protons induced spallation on the ThCx target In this experiment no a-particle which could be assigned to the decay of 180TI have been observed The high energy part of 3 spectrum (45 lt Ep lt 65 MeV) at this mass number was attributed to TI arguing that the 3-endpoint energy for the daughter 18UHg is less than 45 MeV Some 60 counts were detected and a half-life of 19plusmnO9)s was obtained for 18UTI [23] These results cannot be considered as an unambiguous identification of the isotope

7

Figure 3 Possible decay modes for the nucleus 18oTI the decay energies are derived from atomic mass evaluashy

180 tion of ref [22] 80 Hg loo

The aim of the present work was to identify 180TI and study its a-decay properties by using induced fusion-evaporation reactions and mass separation technique of the IGISOL type

In our experiment 144Sm targets enriched to 86 involving also 147Sm (4) were bombarded with 40Call+ beams accelerated by the K=90 cyclotron of the SARA facility We used 3 mgcm2 targets of 144Sm203 on 11 mgcm2 Al backshyings Bombarding energy at mid-target was chosen to be IV 210 MeV according to systematics and statistical-model expectation of the energy position for the maxishymum yield of the (p3n) evaporation channel

Typical intensities of 40Ca beams were in the range 10 to 30 pnA Shown in Figure 4 is the sum a-particle energy spectrum for the mass chain A=180 in case of the 144Sm+40Ca reaction Both the collection and counting time intervals were set to be 2 seconds in this case It is seen here that the well-known 6120 MeV a-line from 180H having a half-life of (256plusmn002)s and an a-decay branching of 35 [24] has been produced and mass-separated The width of the a-line is in agreement with the energy resolution of our SB Si detector (500 pm FWHM=37 keV at 5486 MeV) which was calibrated with standard a-sources (239pu 241 Am 244Cm) In the a-particle energy range around 575 MeV the daughter a-activity of 180Hg is present in full agreement with the a decay properties known for 176Pt(T12=63 5

bo=41 Eo=bo 5750 MeV) [25] The ratio of the peak areas of the two a-lines are in good agreement with the decay properties of the 180Hg --+ 176Pt after taking into account corrections associated with the 2 s collection and 2 s counting times

Further in Figure 4 ana-line of 607plusmn02 MeV is clearly seen the area of which represents about 30 of that of the 6120 MeV a-line of 18degHgThis is a neW a-line in the A=180 chain because it was not observed in the detailed on-line mass-separation studies performed with the 144Sm+40Ar reaction [25J There are also seen in Figure 4 weaker a-groups between 580 and 605 MeV whereas above 615 MeV only very weak a-activites are present

8

III Spectroscopic results

AI Triaxiality in n-rich Ru isotopes In e-e nuclei the spacings observed in sequences of low-lying levels together

with the strengths of electromagnetic transitions are currently used to provide evidence of the rotation of rigid nuclei with various shapes The case of triaxial deformations is of special interest regarding this thirty years old question whether axially asymmetric nuclei are -unstable or rigid triaxial rotors

The systematics study of n-rich e-e Pd Ru Mo and Zr isotopes is of great interest to understand the gradual shape change from spherical vibrator to deformed rotor Our investigations have been focussed on the low-lying collective levels of llU1l2Ru fed by the beta decay of llO112Tc produced via charged particle-induced fission of 238 U and mass-separated using the IGI80L technique described in section II-B In addition to the discovery of the new shortlived (280 ms) 112Tc isotope we have observed in 112Ru one of the lowest second 2+ state ever reported (except in 1920S) Figure 2 shows the summary of the experimentally known low-lying collective levels in n-rich Ru nuclei obtained from our work [17] and previous data [18] Results concerning 114Ru have been obtained in a very recent experiment by measuring prompt -rays in 248Cm fission fragments with the EUROGAM large detector array [19] From this figure it can be seen that near the middle of the N=50 and 82 neutron shells the gs bands (0+2+4+) are rather similar whereas the second 2+ and first 3+ levels which belong to the -band have gradually decreasing excitation energies with a minimum occuring in 112Ru

15

shygtCIJ

--s 1~

~ CIJ c

IlJ

05 2+__ ~ -- +

middotmiddotmiddotmiddotmiddot__________2

o -------------------0+ 102R 104 l06 108 110 112 114

U R U R u Ru Ru Ru Ru

Figure 2 Some low-lying levels of 102-114Ru isotopes

6

On the basis of the two well-known empirical criteria ie the position of the 2t state below the 4t state and the sum relation on the 3t energy it is obvious that llUll2Ru and 114Ru can adopt triaxial shapes in their ground states In terms of the rigid triaxial rotor model of Davydov and Filippov [20] the values (deduced from the energies of the two 2+ levels) are 242deg 264deg and 272deg for A=110 112 and 114 respectively The agreement is also good for B(E2) ratios as was established in ref [17] and more extensively in ref [19] Microscopic lattice Hartree-Fock calculations for 1U8IlU1l2Ru [17] similar to those developed by Bonche et ale [21] have shown that the n-rich Ru are strongly defornled with a quasi degeneracy in the -direction The minima predicted in the valley are too shallow to ensure a rigid axial asymmetry as indicated by the level spacings

It becomes suggestive when comparing the level spectra of n-rich Ru nuclei with the Ba isotones in the valence nucleon scheme that it is the large neutron excess which drives the structure towards soft triaxiality

B Decay properties of very n-deficient TI isotopes Until now there are only very scarce data on the gs decay properties of the

lightest TI isotopes (A lt 184) as compared to extremely n-deficient neighbouring Pb Hg and Au nuclides Besides the evident general interest of exploring properties of isotopes close to the proton drip line there is a number of particular reasons stressing the importance of the determination of the gs decay pattern of 18UTl This nucleus can undergo many decay modes as shown in Figure 3 with estimated energies from ref [22]

First EC-delayed fission was observed and assigned to 180TI [3] precursor Second a-decay properties are poorly known for ground states and isomeric states of TI isotopes with A ~ 183 Third does large deformation as observed in this region for gs of 183185Hg strongly affect decay properties As to 180TI specifically its half-life was deduced from EC-delayed fission studies to be T 12=(97~gg)s [3] In this work the assignment was based on many cross-bombardments and by involving gs decay properties of nuclei in this region as well as systematics of HI-induced fusion-evaporation reactions However it should be corroborated by direct experiments An attempt was performed at the IRIS-faciIity using massshyseparation of products from 1 GeV protons induced spallation on the ThCx target In this experiment no a-particle which could be assigned to the decay of 180TI have been observed The high energy part of 3 spectrum (45 lt Ep lt 65 MeV) at this mass number was attributed to TI arguing that the 3-endpoint energy for the daughter 18UHg is less than 45 MeV Some 60 counts were detected and a half-life of 19plusmnO9)s was obtained for 18UTI [23] These results cannot be considered as an unambiguous identification of the isotope

7

Figure 3 Possible decay modes for the nucleus 18oTI the decay energies are derived from atomic mass evaluashy

180 tion of ref [22] 80 Hg loo

The aim of the present work was to identify 180TI and study its a-decay properties by using induced fusion-evaporation reactions and mass separation technique of the IGISOL type

In our experiment 144Sm targets enriched to 86 involving also 147Sm (4) were bombarded with 40Call+ beams accelerated by the K=90 cyclotron of the SARA facility We used 3 mgcm2 targets of 144Sm203 on 11 mgcm2 Al backshyings Bombarding energy at mid-target was chosen to be IV 210 MeV according to systematics and statistical-model expectation of the energy position for the maxishymum yield of the (p3n) evaporation channel

Typical intensities of 40Ca beams were in the range 10 to 30 pnA Shown in Figure 4 is the sum a-particle energy spectrum for the mass chain A=180 in case of the 144Sm+40Ca reaction Both the collection and counting time intervals were set to be 2 seconds in this case It is seen here that the well-known 6120 MeV a-line from 180H having a half-life of (256plusmn002)s and an a-decay branching of 35 [24] has been produced and mass-separated The width of the a-line is in agreement with the energy resolution of our SB Si detector (500 pm FWHM=37 keV at 5486 MeV) which was calibrated with standard a-sources (239pu 241 Am 244Cm) In the a-particle energy range around 575 MeV the daughter a-activity of 180Hg is present in full agreement with the a decay properties known for 176Pt(T12=63 5

bo=41 Eo=bo 5750 MeV) [25] The ratio of the peak areas of the two a-lines are in good agreement with the decay properties of the 180Hg --+ 176Pt after taking into account corrections associated with the 2 s collection and 2 s counting times

Further in Figure 4 ana-line of 607plusmn02 MeV is clearly seen the area of which represents about 30 of that of the 6120 MeV a-line of 18degHgThis is a neW a-line in the A=180 chain because it was not observed in the detailed on-line mass-separation studies performed with the 144Sm+40Ar reaction [25J There are also seen in Figure 4 weaker a-groups between 580 and 605 MeV whereas above 615 MeV only very weak a-activites are present

8

On the basis of the two well-known empirical criteria ie the position of the 2t state below the 4t state and the sum relation on the 3t energy it is obvious that llUll2Ru and 114Ru can adopt triaxial shapes in their ground states In terms of the rigid triaxial rotor model of Davydov and Filippov [20] the values (deduced from the energies of the two 2+ levels) are 242deg 264deg and 272deg for A=110 112 and 114 respectively The agreement is also good for B(E2) ratios as was established in ref [17] and more extensively in ref [19] Microscopic lattice Hartree-Fock calculations for 1U8IlU1l2Ru [17] similar to those developed by Bonche et ale [21] have shown that the n-rich Ru are strongly defornled with a quasi degeneracy in the -direction The minima predicted in the valley are too shallow to ensure a rigid axial asymmetry as indicated by the level spacings

It becomes suggestive when comparing the level spectra of n-rich Ru nuclei with the Ba isotones in the valence nucleon scheme that it is the large neutron excess which drives the structure towards soft triaxiality

B Decay properties of very n-deficient TI isotopes Until now there are only very scarce data on the gs decay properties of the

lightest TI isotopes (A lt 184) as compared to extremely n-deficient neighbouring Pb Hg and Au nuclides Besides the evident general interest of exploring properties of isotopes close to the proton drip line there is a number of particular reasons stressing the importance of the determination of the gs decay pattern of 18UTl This nucleus can undergo many decay modes as shown in Figure 3 with estimated energies from ref [22]

First EC-delayed fission was observed and assigned to 180TI [3] precursor Second a-decay properties are poorly known for ground states and isomeric states of TI isotopes with A ~ 183 Third does large deformation as observed in this region for gs of 183185Hg strongly affect decay properties As to 180TI specifically its half-life was deduced from EC-delayed fission studies to be T 12=(97~gg)s [3] In this work the assignment was based on many cross-bombardments and by involving gs decay properties of nuclei in this region as well as systematics of HI-induced fusion-evaporation reactions However it should be corroborated by direct experiments An attempt was performed at the IRIS-faciIity using massshyseparation of products from 1 GeV protons induced spallation on the ThCx target In this experiment no a-particle which could be assigned to the decay of 180TI have been observed The high energy part of 3 spectrum (45 lt Ep lt 65 MeV) at this mass number was attributed to TI arguing that the 3-endpoint energy for the daughter 18UHg is less than 45 MeV Some 60 counts were detected and a half-life of 19plusmnO9)s was obtained for 18UTI [23] These results cannot be considered as an unambiguous identification of the isotope

7

Figure 3 Possible decay modes for the nucleus 18oTI the decay energies are derived from atomic mass evaluashy

180 tion of ref [22] 80 Hg loo

The aim of the present work was to identify 180TI and study its a-decay properties by using induced fusion-evaporation reactions and mass separation technique of the IGISOL type

In our experiment 144Sm targets enriched to 86 involving also 147Sm (4) were bombarded with 40Call+ beams accelerated by the K=90 cyclotron of the SARA facility We used 3 mgcm2 targets of 144Sm203 on 11 mgcm2 Al backshyings Bombarding energy at mid-target was chosen to be IV 210 MeV according to systematics and statistical-model expectation of the energy position for the maxishymum yield of the (p3n) evaporation channel

Typical intensities of 40Ca beams were in the range 10 to 30 pnA Shown in Figure 4 is the sum a-particle energy spectrum for the mass chain A=180 in case of the 144Sm+40Ca reaction Both the collection and counting time intervals were set to be 2 seconds in this case It is seen here that the well-known 6120 MeV a-line from 180H having a half-life of (256plusmn002)s and an a-decay branching of 35 [24] has been produced and mass-separated The width of the a-line is in agreement with the energy resolution of our SB Si detector (500 pm FWHM=37 keV at 5486 MeV) which was calibrated with standard a-sources (239pu 241 Am 244Cm) In the a-particle energy range around 575 MeV the daughter a-activity of 180Hg is present in full agreement with the a decay properties known for 176Pt(T12=63 5

bo=41 Eo=bo 5750 MeV) [25] The ratio of the peak areas of the two a-lines are in good agreement with the decay properties of the 180Hg --+ 176Pt after taking into account corrections associated with the 2 s collection and 2 s counting times

Further in Figure 4 ana-line of 607plusmn02 MeV is clearly seen the area of which represents about 30 of that of the 6120 MeV a-line of 18degHgThis is a neW a-line in the A=180 chain because it was not observed in the detailed on-line mass-separation studies performed with the 144Sm+40Ar reaction [25J There are also seen in Figure 4 weaker a-groups between 580 and 605 MeV whereas above 615 MeV only very weak a-activites are present

8

Figure 3 Possible decay modes for the nucleus 18oTI the decay energies are derived from atomic mass evaluashy

180 tion of ref [22] 80 Hg loo

The aim of the present work was to identify 180TI and study its a-decay properties by using induced fusion-evaporation reactions and mass separation technique of the IGISOL type

In our experiment 144Sm targets enriched to 86 involving also 147Sm (4) were bombarded with 40Call+ beams accelerated by the K=90 cyclotron of the SARA facility We used 3 mgcm2 targets of 144Sm203 on 11 mgcm2 Al backshyings Bombarding energy at mid-target was chosen to be IV 210 MeV according to systematics and statistical-model expectation of the energy position for the maxishymum yield of the (p3n) evaporation channel

Typical intensities of 40Ca beams were in the range 10 to 30 pnA Shown in Figure 4 is the sum a-particle energy spectrum for the mass chain A=180 in case of the 144Sm+40Ca reaction Both the collection and counting time intervals were set to be 2 seconds in this case It is seen here that the well-known 6120 MeV a-line from 180H having a half-life of (256plusmn002)s and an a-decay branching of 35 [24] has been produced and mass-separated The width of the a-line is in agreement with the energy resolution of our SB Si detector (500 pm FWHM=37 keV at 5486 MeV) which was calibrated with standard a-sources (239pu 241 Am 244Cm) In the a-particle energy range around 575 MeV the daughter a-activity of 180Hg is present in full agreement with the a decay properties known for 176Pt(T12=63 5

bo=41 Eo=bo 5750 MeV) [25] The ratio of the peak areas of the two a-lines are in good agreement with the decay properties of the 180Hg --+ 176Pt after taking into account corrections associated with the 2 s collection and 2 s counting times

Further in Figure 4 ana-line of 607plusmn02 MeV is clearly seen the area of which represents about 30 of that of the 6120 MeV a-line of 18degHgThis is a neW a-line in the A=180 chain because it was not observed in the detailed on-line mass-separation studies performed with the 144Sm+40Ar reaction [25J There are also seen in Figure 4 weaker a-groups between 580 and 605 MeV whereas above 615 MeV only very weak a-activites are present

8

-

60r-----------__________________________~

50

c 40 ~ tilc =1 30 o

U

20

10

A= 180 collection time = 2s

c 00 c

II

lt -

0 t-shy0 0

J

5500 5750 6000 6250 6500 Alpha energy (ke V)

Figure 4 a-particle spectrum measured at mass A=180 for the reaction 144Sm+ -degCa Collection and counting time intervals were equal to 2 seconds

Obviously no a-line at 607 MeV can be seen in the a-spectrum measured at A=181 mass chain and moreover a possible contamination due to the neighbouring 180Hg can be neglected It is also worth noting that there is no a-activity beyond 615 MeV

The experimental data presented above give good grounds to attribute the 607 MeV a-line to the decay of 180Tl However the energy of this line is too low to fit with the a-decCitY systematics in this region of nuclei It is also worth mentioning the possible a-activity due to 174Pt with similar half-life ( 09 s) and Ea =6038 Me V which could be produced as a result of the 2p-decay of 180Pb

Complementary studies have been carried out at the Dubna U400 cyclotron by employing the Dubna gas-filled recoil separator These bombardment allowed the experimental sensitivity to be increased by about 102 times compared to SARA experiments although by price of replacing mass separation with only kinematic separation The data collected at Dubna are presently under analysis

9

C The region of n-deficient rare-earth nuclei This region of nuclei around A=130 usually referred as a transitional region has

been the subject of many experimental and theoretical investigations At SARA a great number of experiments have been performed using both the He-jet and IGISOL techniques to study the products of 929496Mo+40Ca reactions Several level schemes fed by beta decays have been established in Pr Ce and La isotopes with 127 ~ A ~ 135 and results are given in ref [27]

J ltJ A=127z z laquo Collecting time 1OsI U 100 ~ (I) shyZ t shy shyJ III 0 0

N

050 J - j

bull shyN

r-- co

lt-Q)

~ aq---

0 250 500 750 1000 1250 1500 1750 2000 CHANNEL NUMBER

Figure 5 B-gated I spectrum measured at mass A=127 with lOs collection-and counting-time

Among the recent results the identification of the new isotope 127Pr (4plusmnls) was achieved by measuring the B-gated I spectrum of 127 mass chain (see Figure 5) The study of the 127Pr -+ 127Ce ECB+ decay is worth noting since it enabled us to establish the first excited levels of three collective bands in 127Ceo Two of these bands based on 72- and 52+ levels were already known from previous in-beam studies [28] whereas a third one of positive parity could be associated mainly with the d32 configuration [29]

The systematics presented in Figure 6 exhibits the low-energy levels assigned via our measureshyment in odd-A n-deficient Ce isotopes

Figure 6 Low-lying levels in odd-A Ce isotopes

10

2 0 vro 133 135

The proton drip line localisation is also of utmost interest in this region of the chart It is well known that proton radioactivity which is one of the simplest decay process will determine the limits to nuclear stability for proton-rich nuclei Nine proton emitters have been identified so far near lOOSn and the closed neutron shell N ==82 Taking into account the calculated masses Qp values may be derived and the partial proton half-lives can be calculated in a semi-classical way Moreover from present data it is possible to speculate on the most neutron rich candidates for proton radioactivity between 1091 and 160Re using the formula derived by Hofmann [30] Z==0743 N+116 Along this line with ~N==plusmn 1 (see Figure 7) about twenty nuclei can be considered for experimental investigations

Figure 7 Partial chart of nuclei along the proton drip line from 1091 to 151 Lu

With stable n-deficient targets (92 Mo 96Ru) and intense beams of 36 Ar or 40Ca available at SARA we plan to search for 118La 123Pr 127128Pm and 132Eu which are expected to be produced with rv 100 JLbarn cross-sections via fusion-evaporation reactions But to be observed the p-decay partial half-life value has to be in a rather limited time window due to both ECP+ decay competition and separation time of the IG1S0L technique (gt 1 ms)

11

In addition with the strong dependence on Qp value the importance of angushylar momentum of single-particle proton parent state may affect the partial proton half-life by several orders of magnitude The measurement of the proton energy value is of course of great importance since accurate nuclear masses can be derived and compared with theoretical predictions Search for gs proton radioactivity in the mass 130 region is a challenge for experimentalists since higher sensitivity experiments have to be carried out they are underway at SARA

Summary

The systematic study of the low-lying levels in n-rich Ru isotopes has allowed us to show an axial symmetry breaking whereas complementary investigations are needed to clarify the case of 180TI decay A number of new spectroscopic data such as new isotopes identification have been gained in the region of light rare earth nuclei

The IGISOL technique which has been applied up to now for two types of reactions (fission and fusion-evaporation) has permitted many pioneering investishygations on short-lived nuclei of refractory elements The survival time of the singly charged ions in He ( a few ms) is nowadays a fundamental limitation to the effishycient mass-separation of activities with much shorter half-lives However it is worth noting that this technique is in its infancy and for example its coupling with a laser source could provide in near future both an important increase of efficiency and selective ionization

Acknowledgements

This work was done with the financial support of the Academy of Finland and the PICS-programme of CNRS (France) and also within the framework of the JINR Dubna-IN2P3 (CNRS) collaboration One of the authors (RB) would like to emphasize that the contents of this text and talk have been strongly influenced by the SARA on-line isotope separator team work He would like to express special thanks to M Meyer L Ducroux (IPN Lyon) J Blachot J Inchaouh (ISN Grenoble) A Bouldjedri (Batna University) B Weiss (University of Nice) IN Izosimov (Rad Inst St Petersburg) A Wojtasiewicz (Warsaw University) z Preibisz (INR Swierk) S Chojnacki (Kielce University) G Cata-Danil (IFA Bucharest) A Jokinen ME Leino H Penttila P Taskinen (Jyviiskyla Univershysity) K Eskola (Helsinki University) with the technical support by R Bouvier R Guglielmini G Margotton JP Richaud L Vidal and J L Vieux-Rochaz

12

References

[1] Jauho PP et ale in Proc First European Biennial Workshop on Nuclear Physics Guinet D and Pizzi J R Eds Megeve (France) 1991 p 236 World Scientific

[2] Ayto J et ala Nucl Phys A515 (1990) 365 [3] Lazarev YuA et al Europhys Lett 4 (1987) 893 [4] Genevey J et al Inst Phys Conf Sere 132 (1992) 671 [5] Treherne J et ale Z Phys A309 (1982) 135 [6] PlantierA et ala Nucl Instr amp Meth in Phys Res B26 (1987) 314 [7] Arje J et ala Phys Rev Lett 54 (1985) 99 [8] Arje J et ale Nucl Instr amp Meth in Phys Res A247 (1986) 431 [9] Morita K et ala Nucl Instr amp Meth in Phys Res B26 (1987) 406

[10] Deneffe K et ale Nucl Instr amp Meth in Phys Res B26 (1987) 394 [11] Nomura T et ala Nucl Instr amp Meth in Phys Res A269 (1988) 23 [12] Taskinen P et ala Nucl Instr amp Meth in Phys Res A281 (1989) 539 [13] Astier A et ala Nucl Instr amp Meth in Phys Res B70 (1992) 233 [14] Aysto J et ala Phys Rev Lett 69 (1992) 1167 and ref therein [15] Sprouse GD et ale Phys Rev Lett 63 (1989) 1463 [16] Beraud R et ala Nucl Instr amp Meth in Phys Res A346 (1994) 196 [17] Aysto J et ala Nucl Phys A515 (1990) 365 [18] Stachel J et ala Z Phys A316 (1984) 105 [19] Shannon J A et ala Phys Lett B336 (1994) 136 [20] Davydov AS and Filippov BF Nucl Phys 8 (1958) 237 [21] Bonche P et ala Nucl Phys A443 (1985) 39 [22] Audi G and Wapstra AH Nucl Phys A565 (1993) 1 [23] Bolshakov VA et ala Inst Phys Conf Sera 132 (1992) 743 [24] Hagberg E et ala Nucl Phys A318 (1979) 29 [25] Browne E and Firestone RB Table of Radioactive Isotopes 1986 Ed Shirley

VS [26] Wauters J et ala Z Phys A345 (1993) 21 [27] Genevey J et ale Inst Phys Conf Sere 132 (1992) 671 and ref therein [28] Nyak6 BM et ala Z Phys A334 (1989) 513 [29] Genevey J et ala Contribution to the International Conference on Nuclear

Shapes and Nuclear Structure at Low Excitation Energies Antibes June 1994 France

[30] Hofmann S Proc Int Conf on the Future of Nuclear Spectroscopy Crete Greece 1993 p 255 Eds W Gelletly CA Kalfas R Vlastou S Harissopushylos and D Loukas

13

C The region of n-deficient rare-earth nuclei This region of nuclei around A=130 usually referred as a transitional region has

been the subject of many experimental and theoretical investigations At SARA a great number of experiments have been performed using both the He-jet and IGISOL techniques to study the products of 929496Mo+40Ca reactions Several level schemes fed by beta decays have been established in Pr Ce and La isotopes with 127 ~ A ~ 135 and results are given in ref [27]

J ltJ A=127z z laquo Collecting time 1OsI U 100 ~ (I) shyZ t shy shyJ III 0 0

N

050 J - j

bull shyN

r-- co

lt-Q)

~ aq---

0 250 500 750 1000 1250 1500 1750 2000 CHANNEL NUMBER

Figure 5 B-gated I spectrum measured at mass A=127 with lOs collection-and counting-time

Among the recent results the identification of the new isotope 127Pr (4plusmnls) was achieved by measuring the B-gated I spectrum of 127 mass chain (see Figure 5) The study of the 127Pr -+ 127Ce ECB+ decay is worth noting since it enabled us to establish the first excited levels of three collective bands in 127Ceo Two of these bands based on 72- and 52+ levels were already known from previous in-beam studies [28] whereas a third one of positive parity could be associated mainly with the d32 configuration [29]

The systematics presented in Figure 6 exhibits the low-energy levels assigned via our measureshyment in odd-A n-deficient Ce isotopes

Figure 6 Low-lying levels in odd-A Ce isotopes

10

2 0 vro 133 135

The proton drip line localisation is also of utmost interest in this region of the chart It is well known that proton radioactivity which is one of the simplest decay process will determine the limits to nuclear stability for proton-rich nuclei Nine proton emitters have been identified so far near lOOSn and the closed neutron shell N ==82 Taking into account the calculated masses Qp values may be derived and the partial proton half-lives can be calculated in a semi-classical way Moreover from present data it is possible to speculate on the most neutron rich candidates for proton radioactivity between 1091 and 160Re using the formula derived by Hofmann [30] Z==0743 N+116 Along this line with ~N==plusmn 1 (see Figure 7) about twenty nuclei can be considered for experimental investigations

Figure 7 Partial chart of nuclei along the proton drip line from 1091 to 151 Lu

With stable n-deficient targets (92 Mo 96Ru) and intense beams of 36 Ar or 40Ca available at SARA we plan to search for 118La 123Pr 127128Pm and 132Eu which are expected to be produced with rv 100 JLbarn cross-sections via fusion-evaporation reactions But to be observed the p-decay partial half-life value has to be in a rather limited time window due to both ECP+ decay competition and separation time of the IG1S0L technique (gt 1 ms)

11

In addition with the strong dependence on Qp value the importance of angushylar momentum of single-particle proton parent state may affect the partial proton half-life by several orders of magnitude The measurement of the proton energy value is of course of great importance since accurate nuclear masses can be derived and compared with theoretical predictions Search for gs proton radioactivity in the mass 130 region is a challenge for experimentalists since higher sensitivity experiments have to be carried out they are underway at SARA

Summary

The systematic study of the low-lying levels in n-rich Ru isotopes has allowed us to show an axial symmetry breaking whereas complementary investigations are needed to clarify the case of 180TI decay A number of new spectroscopic data such as new isotopes identification have been gained in the region of light rare earth nuclei

The IGISOL technique which has been applied up to now for two types of reactions (fission and fusion-evaporation) has permitted many pioneering investishygations on short-lived nuclei of refractory elements The survival time of the singly charged ions in He ( a few ms) is nowadays a fundamental limitation to the effishycient mass-separation of activities with much shorter half-lives However it is worth noting that this technique is in its infancy and for example its coupling with a laser source could provide in near future both an important increase of efficiency and selective ionization

Acknowledgements

This work was done with the financial support of the Academy of Finland and the PICS-programme of CNRS (France) and also within the framework of the JINR Dubna-IN2P3 (CNRS) collaboration One of the authors (RB) would like to emphasize that the contents of this text and talk have been strongly influenced by the SARA on-line isotope separator team work He would like to express special thanks to M Meyer L Ducroux (IPN Lyon) J Blachot J Inchaouh (ISN Grenoble) A Bouldjedri (Batna University) B Weiss (University of Nice) IN Izosimov (Rad Inst St Petersburg) A Wojtasiewicz (Warsaw University) z Preibisz (INR Swierk) S Chojnacki (Kielce University) G Cata-Danil (IFA Bucharest) A Jokinen ME Leino H Penttila P Taskinen (Jyviiskyla Univershysity) K Eskola (Helsinki University) with the technical support by R Bouvier R Guglielmini G Margotton JP Richaud L Vidal and J L Vieux-Rochaz

12

References

[1] Jauho PP et ale in Proc First European Biennial Workshop on Nuclear Physics Guinet D and Pizzi J R Eds Megeve (France) 1991 p 236 World Scientific

[2] Ayto J et ala Nucl Phys A515 (1990) 365 [3] Lazarev YuA et al Europhys Lett 4 (1987) 893 [4] Genevey J et al Inst Phys Conf Sere 132 (1992) 671 [5] Treherne J et ale Z Phys A309 (1982) 135 [6] PlantierA et ala Nucl Instr amp Meth in Phys Res B26 (1987) 314 [7] Arje J et ala Phys Rev Lett 54 (1985) 99 [8] Arje J et ale Nucl Instr amp Meth in Phys Res A247 (1986) 431 [9] Morita K et ala Nucl Instr amp Meth in Phys Res B26 (1987) 406

[10] Deneffe K et ale Nucl Instr amp Meth in Phys Res B26 (1987) 394 [11] Nomura T et ala Nucl Instr amp Meth in Phys Res A269 (1988) 23 [12] Taskinen P et ala Nucl Instr amp Meth in Phys Res A281 (1989) 539 [13] Astier A et ala Nucl Instr amp Meth in Phys Res B70 (1992) 233 [14] Aysto J et ala Phys Rev Lett 69 (1992) 1167 and ref therein [15] Sprouse GD et ale Phys Rev Lett 63 (1989) 1463 [16] Beraud R et ala Nucl Instr amp Meth in Phys Res A346 (1994) 196 [17] Aysto J et ala Nucl Phys A515 (1990) 365 [18] Stachel J et ala Z Phys A316 (1984) 105 [19] Shannon J A et ala Phys Lett B336 (1994) 136 [20] Davydov AS and Filippov BF Nucl Phys 8 (1958) 237 [21] Bonche P et ala Nucl Phys A443 (1985) 39 [22] Audi G and Wapstra AH Nucl Phys A565 (1993) 1 [23] Bolshakov VA et ala Inst Phys Conf Sera 132 (1992) 743 [24] Hagberg E et ala Nucl Phys A318 (1979) 29 [25] Browne E and Firestone RB Table of Radioactive Isotopes 1986 Ed Shirley

VS [26] Wauters J et ala Z Phys A345 (1993) 21 [27] Genevey J et ale Inst Phys Conf Sere 132 (1992) 671 and ref therein [28] Nyak6 BM et ala Z Phys A334 (1989) 513 [29] Genevey J et ala Contribution to the International Conference on Nuclear

Shapes and Nuclear Structure at Low Excitation Energies Antibes June 1994 France

[30] Hofmann S Proc Int Conf on the Future of Nuclear Spectroscopy Crete Greece 1993 p 255 Eds W Gelletly CA Kalfas R Vlastou S Harissopushylos and D Loukas

13

The proton drip line localisation is also of utmost interest in this region of the chart It is well known that proton radioactivity which is one of the simplest decay process will determine the limits to nuclear stability for proton-rich nuclei Nine proton emitters have been identified so far near lOOSn and the closed neutron shell N ==82 Taking into account the calculated masses Qp values may be derived and the partial proton half-lives can be calculated in a semi-classical way Moreover from present data it is possible to speculate on the most neutron rich candidates for proton radioactivity between 1091 and 160Re using the formula derived by Hofmann [30] Z==0743 N+116 Along this line with ~N==plusmn 1 (see Figure 7) about twenty nuclei can be considered for experimental investigations

Figure 7 Partial chart of nuclei along the proton drip line from 1091 to 151 Lu

With stable n-deficient targets (92 Mo 96Ru) and intense beams of 36 Ar or 40Ca available at SARA we plan to search for 118La 123Pr 127128Pm and 132Eu which are expected to be produced with rv 100 JLbarn cross-sections via fusion-evaporation reactions But to be observed the p-decay partial half-life value has to be in a rather limited time window due to both ECP+ decay competition and separation time of the IG1S0L technique (gt 1 ms)

11

In addition with the strong dependence on Qp value the importance of angushylar momentum of single-particle proton parent state may affect the partial proton half-life by several orders of magnitude The measurement of the proton energy value is of course of great importance since accurate nuclear masses can be derived and compared with theoretical predictions Search for gs proton radioactivity in the mass 130 region is a challenge for experimentalists since higher sensitivity experiments have to be carried out they are underway at SARA

Summary

The systematic study of the low-lying levels in n-rich Ru isotopes has allowed us to show an axial symmetry breaking whereas complementary investigations are needed to clarify the case of 180TI decay A number of new spectroscopic data such as new isotopes identification have been gained in the region of light rare earth nuclei

The IGISOL technique which has been applied up to now for two types of reactions (fission and fusion-evaporation) has permitted many pioneering investishygations on short-lived nuclei of refractory elements The survival time of the singly charged ions in He ( a few ms) is nowadays a fundamental limitation to the effishycient mass-separation of activities with much shorter half-lives However it is worth noting that this technique is in its infancy and for example its coupling with a laser source could provide in near future both an important increase of efficiency and selective ionization

Acknowledgements

This work was done with the financial support of the Academy of Finland and the PICS-programme of CNRS (France) and also within the framework of the JINR Dubna-IN2P3 (CNRS) collaboration One of the authors (RB) would like to emphasize that the contents of this text and talk have been strongly influenced by the SARA on-line isotope separator team work He would like to express special thanks to M Meyer L Ducroux (IPN Lyon) J Blachot J Inchaouh (ISN Grenoble) A Bouldjedri (Batna University) B Weiss (University of Nice) IN Izosimov (Rad Inst St Petersburg) A Wojtasiewicz (Warsaw University) z Preibisz (INR Swierk) S Chojnacki (Kielce University) G Cata-Danil (IFA Bucharest) A Jokinen ME Leino H Penttila P Taskinen (Jyviiskyla Univershysity) K Eskola (Helsinki University) with the technical support by R Bouvier R Guglielmini G Margotton JP Richaud L Vidal and J L Vieux-Rochaz

12

References

[1] Jauho PP et ale in Proc First European Biennial Workshop on Nuclear Physics Guinet D and Pizzi J R Eds Megeve (France) 1991 p 236 World Scientific

[2] Ayto J et ala Nucl Phys A515 (1990) 365 [3] Lazarev YuA et al Europhys Lett 4 (1987) 893 [4] Genevey J et al Inst Phys Conf Sere 132 (1992) 671 [5] Treherne J et ale Z Phys A309 (1982) 135 [6] PlantierA et ala Nucl Instr amp Meth in Phys Res B26 (1987) 314 [7] Arje J et ala Phys Rev Lett 54 (1985) 99 [8] Arje J et ale Nucl Instr amp Meth in Phys Res A247 (1986) 431 [9] Morita K et ala Nucl Instr amp Meth in Phys Res B26 (1987) 406

[10] Deneffe K et ale Nucl Instr amp Meth in Phys Res B26 (1987) 394 [11] Nomura T et ala Nucl Instr amp Meth in Phys Res A269 (1988) 23 [12] Taskinen P et ala Nucl Instr amp Meth in Phys Res A281 (1989) 539 [13] Astier A et ala Nucl Instr amp Meth in Phys Res B70 (1992) 233 [14] Aysto J et ala Phys Rev Lett 69 (1992) 1167 and ref therein [15] Sprouse GD et ale Phys Rev Lett 63 (1989) 1463 [16] Beraud R et ala Nucl Instr amp Meth in Phys Res A346 (1994) 196 [17] Aysto J et ala Nucl Phys A515 (1990) 365 [18] Stachel J et ala Z Phys A316 (1984) 105 [19] Shannon J A et ala Phys Lett B336 (1994) 136 [20] Davydov AS and Filippov BF Nucl Phys 8 (1958) 237 [21] Bonche P et ala Nucl Phys A443 (1985) 39 [22] Audi G and Wapstra AH Nucl Phys A565 (1993) 1 [23] Bolshakov VA et ala Inst Phys Conf Sera 132 (1992) 743 [24] Hagberg E et ala Nucl Phys A318 (1979) 29 [25] Browne E and Firestone RB Table of Radioactive Isotopes 1986 Ed Shirley

VS [26] Wauters J et ala Z Phys A345 (1993) 21 [27] Genevey J et ale Inst Phys Conf Sere 132 (1992) 671 and ref therein [28] Nyak6 BM et ala Z Phys A334 (1989) 513 [29] Genevey J et ala Contribution to the International Conference on Nuclear

Shapes and Nuclear Structure at Low Excitation Energies Antibes June 1994 France

[30] Hofmann S Proc Int Conf on the Future of Nuclear Spectroscopy Crete Greece 1993 p 255 Eds W Gelletly CA Kalfas R Vlastou S Harissopushylos and D Loukas

13

In addition with the strong dependence on Qp value the importance of angushylar momentum of single-particle proton parent state may affect the partial proton half-life by several orders of magnitude The measurement of the proton energy value is of course of great importance since accurate nuclear masses can be derived and compared with theoretical predictions Search for gs proton radioactivity in the mass 130 region is a challenge for experimentalists since higher sensitivity experiments have to be carried out they are underway at SARA

Summary

The systematic study of the low-lying levels in n-rich Ru isotopes has allowed us to show an axial symmetry breaking whereas complementary investigations are needed to clarify the case of 180TI decay A number of new spectroscopic data such as new isotopes identification have been gained in the region of light rare earth nuclei

The IGISOL technique which has been applied up to now for two types of reactions (fission and fusion-evaporation) has permitted many pioneering investishygations on short-lived nuclei of refractory elements The survival time of the singly charged ions in He ( a few ms) is nowadays a fundamental limitation to the effishycient mass-separation of activities with much shorter half-lives However it is worth noting that this technique is in its infancy and for example its coupling with a laser source could provide in near future both an important increase of efficiency and selective ionization

Acknowledgements

This work was done with the financial support of the Academy of Finland and the PICS-programme of CNRS (France) and also within the framework of the JINR Dubna-IN2P3 (CNRS) collaboration One of the authors (RB) would like to emphasize that the contents of this text and talk have been strongly influenced by the SARA on-line isotope separator team work He would like to express special thanks to M Meyer L Ducroux (IPN Lyon) J Blachot J Inchaouh (ISN Grenoble) A Bouldjedri (Batna University) B Weiss (University of Nice) IN Izosimov (Rad Inst St Petersburg) A Wojtasiewicz (Warsaw University) z Preibisz (INR Swierk) S Chojnacki (Kielce University) G Cata-Danil (IFA Bucharest) A Jokinen ME Leino H Penttila P Taskinen (Jyviiskyla Univershysity) K Eskola (Helsinki University) with the technical support by R Bouvier R Guglielmini G Margotton JP Richaud L Vidal and J L Vieux-Rochaz

12

References

[1] Jauho PP et ale in Proc First European Biennial Workshop on Nuclear Physics Guinet D and Pizzi J R Eds Megeve (France) 1991 p 236 World Scientific

[2] Ayto J et ala Nucl Phys A515 (1990) 365 [3] Lazarev YuA et al Europhys Lett 4 (1987) 893 [4] Genevey J et al Inst Phys Conf Sere 132 (1992) 671 [5] Treherne J et ale Z Phys A309 (1982) 135 [6] PlantierA et ala Nucl Instr amp Meth in Phys Res B26 (1987) 314 [7] Arje J et ala Phys Rev Lett 54 (1985) 99 [8] Arje J et ale Nucl Instr amp Meth in Phys Res A247 (1986) 431 [9] Morita K et ala Nucl Instr amp Meth in Phys Res B26 (1987) 406

[10] Deneffe K et ale Nucl Instr amp Meth in Phys Res B26 (1987) 394 [11] Nomura T et ala Nucl Instr amp Meth in Phys Res A269 (1988) 23 [12] Taskinen P et ala Nucl Instr amp Meth in Phys Res A281 (1989) 539 [13] Astier A et ala Nucl Instr amp Meth in Phys Res B70 (1992) 233 [14] Aysto J et ala Phys Rev Lett 69 (1992) 1167 and ref therein [15] Sprouse GD et ale Phys Rev Lett 63 (1989) 1463 [16] Beraud R et ala Nucl Instr amp Meth in Phys Res A346 (1994) 196 [17] Aysto J et ala Nucl Phys A515 (1990) 365 [18] Stachel J et ala Z Phys A316 (1984) 105 [19] Shannon J A et ala Phys Lett B336 (1994) 136 [20] Davydov AS and Filippov BF Nucl Phys 8 (1958) 237 [21] Bonche P et ala Nucl Phys A443 (1985) 39 [22] Audi G and Wapstra AH Nucl Phys A565 (1993) 1 [23] Bolshakov VA et ala Inst Phys Conf Sera 132 (1992) 743 [24] Hagberg E et ala Nucl Phys A318 (1979) 29 [25] Browne E and Firestone RB Table of Radioactive Isotopes 1986 Ed Shirley

VS [26] Wauters J et ala Z Phys A345 (1993) 21 [27] Genevey J et ale Inst Phys Conf Sere 132 (1992) 671 and ref therein [28] Nyak6 BM et ala Z Phys A334 (1989) 513 [29] Genevey J et ala Contribution to the International Conference on Nuclear

Shapes and Nuclear Structure at Low Excitation Energies Antibes June 1994 France

[30] Hofmann S Proc Int Conf on the Future of Nuclear Spectroscopy Crete Greece 1993 p 255 Eds W Gelletly CA Kalfas R Vlastou S Harissopushylos and D Loukas

13

References

[1] Jauho PP et ale in Proc First European Biennial Workshop on Nuclear Physics Guinet D and Pizzi J R Eds Megeve (France) 1991 p 236 World Scientific

[2] Ayto J et ala Nucl Phys A515 (1990) 365 [3] Lazarev YuA et al Europhys Lett 4 (1987) 893 [4] Genevey J et al Inst Phys Conf Sere 132 (1992) 671 [5] Treherne J et ale Z Phys A309 (1982) 135 [6] PlantierA et ala Nucl Instr amp Meth in Phys Res B26 (1987) 314 [7] Arje J et ala Phys Rev Lett 54 (1985) 99 [8] Arje J et ale Nucl Instr amp Meth in Phys Res A247 (1986) 431 [9] Morita K et ala Nucl Instr amp Meth in Phys Res B26 (1987) 406

[10] Deneffe K et ale Nucl Instr amp Meth in Phys Res B26 (1987) 394 [11] Nomura T et ala Nucl Instr amp Meth in Phys Res A269 (1988) 23 [12] Taskinen P et ala Nucl Instr amp Meth in Phys Res A281 (1989) 539 [13] Astier A et ala Nucl Instr amp Meth in Phys Res B70 (1992) 233 [14] Aysto J et ala Phys Rev Lett 69 (1992) 1167 and ref therein [15] Sprouse GD et ale Phys Rev Lett 63 (1989) 1463 [16] Beraud R et ala Nucl Instr amp Meth in Phys Res A346 (1994) 196 [17] Aysto J et ala Nucl Phys A515 (1990) 365 [18] Stachel J et ala Z Phys A316 (1984) 105 [19] Shannon J A et ala Phys Lett B336 (1994) 136 [20] Davydov AS and Filippov BF Nucl Phys 8 (1958) 237 [21] Bonche P et ala Nucl Phys A443 (1985) 39 [22] Audi G and Wapstra AH Nucl Phys A565 (1993) 1 [23] Bolshakov VA et ala Inst Phys Conf Sera 132 (1992) 743 [24] Hagberg E et ala Nucl Phys A318 (1979) 29 [25] Browne E and Firestone RB Table of Radioactive Isotopes 1986 Ed Shirley

VS [26] Wauters J et ala Z Phys A345 (1993) 21 [27] Genevey J et ale Inst Phys Conf Sere 132 (1992) 671 and ref therein [28] Nyak6 BM et ala Z Phys A334 (1989) 513 [29] Genevey J et ala Contribution to the International Conference on Nuclear

Shapes and Nuclear Structure at Low Excitation Energies Antibes June 1994 France

[30] Hofmann S Proc Int Conf on the Future of Nuclear Spectroscopy Crete Greece 1993 p 255 Eds W Gelletly CA Kalfas R Vlastou S Harissopushylos and D Loukas

13